1. Field of the Invention
The present invention particularly relates to an organic electroluminescence (EL) display or other image display devices comprised of pixel circuits, having electro-optical elements whose luminance is controlled by a current value, arranged in a matrix, in particular a so-called active matrix type image display device in which a value of a current flowing through an electro-optical element is controlled by an insulating gate type field effect transistor provided inside each pixel circuit.
2. Description of the Related Art
In an image display device, for example, a liquid crystal display, an image is displayed by arranging a large number of pixels in a matrix and controlling a light intensity for every pixel in accordance with image information to be displayed. The same is true for an organic EL display etc., but an organic EL display is a so-called self light emitting type display which has light emitting elements in the pixel circuits and has the advantages that the viewability is high in comparison with a liquid crystal display, no backlight is required, a response speed is high, etc. Further, it greatly differs from a liquid crystal display etc. in the point that the luminance of each light emitting element is controlled by the value of the current flowing through it to give tones of the emitted colors, that is, the light emitting elements are current controlled types.
An organic EL display, in the same way as a liquid crystal display, may be driven by the simple matrix system and the active matrix system, but while the former is simple in structure, it has problems such as the difficulty of realization of a large scale and high definition display. For this reason, there has been active development of the active matrix system controlling the current flowing through the light emitting element inside each pixel circuit by an active element provided inside the pixel circuit, generally, a thin film transistor (TFT).
FIG. 1 is a block diagram of the configuration of a general organic EL display device. This display device 1 has, as shown in FIG. 1, a pixel array 2 having pixel circuits (PXLC) 2a arranged in an m×n matrix, a horizontal selector (HSEL) 3, a write scanner (WSCN) 4, data lines DTL1 to DTLn selected by the horizontal selector 3 and supplied with data signals in accordance with the luminance information, and scanning lines WSL1 to WSLm selectively driven by the write scanner 4.
FIG. 2 is a circuit diagram of an example of the configuration of a pixel circuit 2a of FIG. 1 (refer to for example U.S. Pat. No. 5,684,365 and Japanese Unexamined Patent Publication (Kokai) No. 8-234683. The pixel circuit of FIG. 2 has the simplest circuit configuration among the many circuits which have been proposed and is a so-called two-transistor drive type circuit.
The pixel circuit 2a of FIG. 2 has a p-channel thin film field effect transistor (hereinafter, referred to as a “TFT”) 11 and a TFT 12, a capacitor C11, and a light emitting element 13 constituted by an organic EL element. Further, in FIG. 2, DTL indicates a data line, and WSL indicates a scanning line. An organic EL element has a rectification property in many cases, so is sometimes referred to as an “organic light emitting diode” (OLED). The symbol of a diode is used for the light emitting element in FIG. 2 and other figures, but a rectification property is not always required for the organic EL element in the following explanation. In FIG. 2, the source of the TFT 11 is connected to a power supply potential Vcc (supply line of power supply voltage Vcc), and the cathode of the light emitting element 13 is connected to a ground GND. The pixel circuit 2a of FIG. 2 operates as follows.
Step ST1
When the scanning line WSL is made the selected state (low level here) and a write potential Vdata is applied to the data line DTL, the TFT 12 becomes conductive, the capacitor C11 is charged or discharged, and the gate potential of the TFT 11 becomes Vdata.
Step ST2
When the scanning line WSL is made the nonselected state (high level here), the data line DTL and the TFT 11 are electrically disconnected, but the gate potential of the TFT 11 is stably held by the capacitor C11.
Step ST3
The current flowing through the TFT 11 and the light emitting element 13 becomes a value in accordance with a gate-source voltage Vgs of the TFT 11. The light emitting element 13 continuously emits light with a luminance in accordance with the current value.
The operation of selecting the scanning line WSL and transferring the luminance information given to the data line to the interior of the pixel as in above step ST1 will be referred to as a “write operation” below. As explained above, in the pixel circuit 2a of FIG. 2, when once writing Vdata, the light emitting element 13 continues emitting light with a constant luminance in the period up to when next rewritten.
As explained above, in the pixel circuit 2a, by changing a gate application voltage of the drive transistor constituted by the TFT 11, the value of the current flowing through the light emitting element 13 is controlled. At this time, the source of the drive transistor of p-channel is connected to the power supply potential Vcc. This TFT 11 always operates in a saturated region. Accordingly, it becomes a constant current source having a value shown in equation 1.Ids=1/2·μ(W/L)Cox(Vgs−|Vth|)2   (1)
where, μ indicates the mobility of the carriers, Cox indicates a gate capacitance per unit area, W indicates a gate width, L indicates a gate length, Vgs indicates the gate-source voltage of the TFT 11, and Vth indicates the threshold value Vth of the TFT 11.
In a simple matrix type image display device, each light emitting element emits light only at a selected instant, while in an active matrix type, as explained above, each light emitting element continues to emit light even after the end of the write operation. Therefore, this type becomes advantageous, especially in a large sized, high definition display, in the point that the peak luminance and the peak current of the light emitting elements can be lowered in comparison with the simple matrix type.
However, a TFT generally has a large variation in Vth and mobility μ. For this reason, even if the same input voltage is applied to gates of different drive transistors, the ON currents will vary. As a result, the uniformity of image quality ends up deteriorating.
A large number of pixel circuits have been proposed in order to solve this problem. A representative example is shown in FIG. 3 (refer to for example U.S. Pat. No. 6,229,506 and FIG. 3 of Japanese Unexamined Patent Publication (Kohyo) No. 2002-514320).
A pixel circuit 2b of FIG. 3 has p-channel TFT 21 to TFT 24, capacitors C21 and C22, and a light emitting element 25 constituted by an organic light emitting diode (OLED) 25. Further, in FIG. 3, DTL indicates a data line, WSL indicates a scanning line, AZL indicates an auto-zero line, and DSL indicates a drive line.
The operation of this pixel circuit 2b will be explained below while referring to timing charts shown in FIGS. 4A to 4G. FIG. 4A shows a scanning signal ws[1] applied to a scanning line WSL1 of the first row of the pixel array; FIG. 4B shows a scanning signal ws[2] applied to a scanning line WSL2 of the second row of the pixel array; FIG. 4C shows an auto-zero signal az[1] applied to an auto-zero line AZL1 of the first row of the pixel array; FIG. 4D shows an auto-zero signal az[2] applied to an auto-zero line AZL2 of the second row of the pixel array; FIG. 4E shows a drive signal ds[1] applied to a drive line DSL1 of the first row of the pixel array; FIG. 4F shows a drive signal ds[2] applied to a drive line DSL2 of the second row of the pixel array; and FIG. 4G shows a gate potential Vg of the TFT 21. Note that, in the following description, the operation of the pixel circuit of the first row will be explained.
As shown in FIGS. 4C and 4E, the drive signal ds[1] to the drive line DSL1 and the auto-zero signal az[1] to the auto-zero line AZL1 are made the low level, and the TFT 22 and the TFT 23 are made the conductive state. At this time, the TFT 21 is connected to the light emitting element (OLED) 25 in a diode-connected state, so the current flows through the TFT 21. At this time, the gate potential Vg of the TFT 21 falls as shown in FIG. 4G.
As shown in FIG. 4E, the drive signal ds[1] to the drive line DSL1 is made the high level, and the TFT 22 is made the nonconductive state. At this time, the scanning signal ws[1] to the scanning line WSL1 is the high level and the TFT 24 is held in the nonconductive state as shown in FIG. 4A. Along with the TFT 22 becoming the nonconductive state, the current flowing through the light emitting element 25 is cut off, therefore, as shown in FIG. 4G, the gate potential Vg of the TFT 21 rises, but the TFT 21 becomes the nonconductive state at the point of time when the potential rises up to Vcc−|Vth| and therefore the potential becomes stable. This operation will be referred to as an “auto-zero operation”.
As shown in FIG. 4C, after the auto-zero signal az[1] to the auto-zero line AZL1 is made the high level, the TFT 23 is made the nonconductive state, and the auto-zero operation (Vth correction operation) is terminated, the drive signal ds[1] to the drive line DSL1 is made the low level and the TFT 22 is made the conductive state.
Then, the scanning signal ws[1] to the scanning line WSL1 is made the low level to make the TFT 24 the conductive state as shown in FIG. 4A and thereby apply the data signal of a predetermined potential propagated to the data line DTL1 to the capacitor C21. Due to this, as shown in FIG. 4G, the gate potential of the TFT 21 is lowered by exactly ΔVg via the capacitor C21. As shown in FIG. 4A, the TFT 24 is made the nonconductive state by making the scanning line WSL1 the high level. Due to this, the current flows through the TFT 21 and the light emitting element (OLED) 25, so the light emitting element 25 starts emitting light.
Summarizing the problems to be solved by the invention, as mentioned above, in the pixel circuit of FIG. 3, by turning on the auto-zero switch constituted by the TFT 23 while the light emitting element 25 is not emitting light, the drive transistor TFT 21 is cut off. In the cut off state, current does not flow through this TFT 21, so the gate-source voltage Vgs becomes equal to the threshold value Vth of the transistor. Due to this, variation in the Vth for each pixel is cancelled. Next, by turning off the TFT 23, then turning on the TFT 24, the voltage ΔV is coupled with the data line voltage at the gate of the drive transistor TFT 21 through the capacitor C21 in the pixel. When this coupling amount is V0, the drive transistor TFT 21 carries an ON current corresponding to Vgs−Vth=V0 regardless of the Vth and therefore an image quality without unevenness in uniformity due to variation in the Vth is obtained.
In the pixel circuit of FIG. 3, however, even if the variation in Vth can be corrected, the variation of the mobility p cannot be corrected. Below, this problem will be explained in further detail in relation to the drawings.
FIG. 5 is a graph showing characteristic curves of ΔV (=Vgs−Vth) and the drain-source current Ids of drive transistors having different mobilities in the pixel circuits of FIG. 3. In FIG. 5, an abscissa represents the voltage ΔV, and an ordinate represents the current Ids. Further, in FIG. 5, the curve indicated by the solid line shows the characteristic of a pixel A, and the curve indicated by a broken line shows the characteristic of a pixel B.
As shown in FIG. 5, the mobility differs between the characteristic of the pixel A indicated by the solid line and the characteristic of the pixel B indicated by the broken line. In the pixel circuit system of FIG. 3, at the auto-zero point (ΔV=V0), the current value is equal even between pixel transistors having different mobilities. However, as the voltage rises thereafter, the variation of the mobility μ ends up appearing in the current value. For example, in the pixel A and the pixel B having different mobilities, even when the same voltage ΔV=V0 is applied, variation of the current Ids occurs according to equation 1, so the luminances of the pixels end up differing. That is, as more current flows and the luminance increases, the current value ends up being affected by the variation of the mobility, the uniformity declines, and therefore the image quality ends up deteriorating.
FIG. 6 is a graph of the change of the gate voltage of the drive transistor at the time of an auto-zero operation of pixels C and D having different drive transistor threshold values Vth. In FIG. 6, the abscissa represents a time t, and the ordinate represents the gate voltage Vg. Further, in FIG. 6, the curve indicated by the solid line shows the characteristic of the pixel C, and the curve indicated by the broken line shows the characteristic of the pixel D.
The auto-zero operation is carried out by connecting the gate and the source of the drive transistor, but the closer to the cutoff region, the more rapidly the ON current decreases. For this reason, a long time is required until the cut off is completed and the variation of the threshold value is cancelled out. As shown in FIG. 6, if the auto-zero time is insufficient, the variation of the threshold value Vth is not completely cancelled in the pixel C. In this way, it is believed that due to the variation of the threshold values Vth, the writing state of the gate voltage also varies and the uniformity deteriorates due to this as well.
Further, even if sufficient auto-zero time is taken and the variation of the threshold values Vth is cancelled out, an off current, though small, ends up flowing through the drive transistor after the cutoff. For this reason, as shown in FIG. 7, the gate voltage ends up gradually rising toward the power supply voltage Vcc. As a result, irrespective of the fact that the variation of the threshold values Vth is once cancelled out by the auto-zero operation, in the end, the gate potentials of the pixels with the varied threshold values Vth head toward the power supply voltage, so the variation of the threshold values Vth appears again.
From the above, in an actual device, in order to effectively cancel out variation of the threshold values Vth, it is necessary to optimally adjust the auto-zero period for every panel. Optimum adjustment of the auto-zero period for every panel, however, would require an enormous adjustment time and would end up increasing the cost of the panels.